NOVA

(dramatic music) So let's take a look at the sequence of events that happened before and during this huge earthquake off Japan. The green dot you're looking at behind me was the location of a magnitude 7.2 earthquake that occurred on Wednesday morning, local time, off the coast of Japan, itself a fairly substantial earthquake. That earthquake was followed by a set of smaller earthquakes, aftershocks. And those aftershocks, if we had just seen the sequence, we would have set out just a normal main shock, after shock sequence, but boom, about 50 hours later, there was the main shock, which is shown on this map as the red dot, and then a whole swarm of aftershocks that followed the main shock. And what you're seeing here is how those aftershocks happened over a period of about a week after the main shock, and that orange region delineated by those orange dots, essentially gives you a feeling for the area of the fault along the plate boundary that ruptured in this event. Remember, it's a dipping fault, so it's got some length to it, about 500 kilometers and some width, probably 150 kilometers down the dip of that megathrust zone. And we can expect to see as the sequence unfolds, over the next period of weeks to months and in fact years, that we will get more and more earthquakes as part of the sequence, essentially as aftershocks of this major event. (dramatic music)